1. Field of the Invention
The present invention relates to a control circuit, and more particularly, to a control signal storing circuit for automatic color control (ACC) of video cassette recorders (VCR).
2. Description of the Prior Art
In ACC circuits for VCRs, as is conventionally known, two ACC control signal generating paths are provided which are associated with reproductions by two rotary heads since reproduction levels of the two rotary heads are different (see periods T1 and T2 in FIG. 2A). Outputs of the paths are alternately changed-over in synchronism with a changeover of the two heads and applied to a gain variable amplifier for color signal amplification. At this time, since a level detection output of an input color burst is supplied to a capacitor provided to each path for storing an output of a reproduction color burst level detection (the capacitor is included in a detection circuit) only every other field in correspondence to the changeover of the two heads, the capacitor is in a discharge state during the field period when no input signal is supplied to the capacitor (in other words, during a field period when an output of the capacitor is not used). Because of this, responsiveness deteriorates according to the amount of the discharge of potential when the next input signal is supplied to the capacitor.
Although no problems arise if the capacity of the capacitor is decreased to previously improve responsiveness, since a time constant is comparatively high to remove an effect by noise on a color signal in the color burst detection at the time of normal reproduction, the level of a color signal, which has been controlled by ACC signal, does not promptly reach a desired value at a, b, and c in FIG. 2B,--that is, immediately after the changeover by the head switch. The deterioration of responsiveness at the time of the changeover is more remarkable if the capacitor is discharged during the period when an output of the capacitor is not used.
To solve the above-described problem, Japanese laid-open Utility Model Application H2-53682 proposes that buffer on the output side of each capacitor is cut off during the period when an output of the capacitor is not used. That is, as shown in FIG. 1, in this prior art, to prevent the discharge of the capacitor, an output of a detection circuit 51 for level detection of a reproduction color burst of one (head A) of the two heads is charged into a capacitor 53 to be stored therein and an output of a detection circuit 52 for level detection of a reproduction color burst of the other head (head B) is charged into a capacitor 54 to be stored therein, and switches 57 and 58 are parallely provided between the collector and emitter of output emitter follower transistors (buffers) 55 and 56 connected to the capacitors 53 and 54. By the switches 57 and 58, an emitter follower transistor corresponding to a capacitor whose output is not used is disabled.
In this prior art, however, only discharge of the capacitors 53 and 54 through output paths of the capacitors 53 and 54 is prevented, and prevention of discharge of the capacitors 53 and 54 caused by a leak is not considered.
The discharge by a leak can be prevented if the capacities of the capacitors 53 and 54 are fairly increased. However, since it is impossible to take a high capacity value when the capacitors 53 and 54 are formed in an integrated circuit (IC), a decrease in voltage of the capacitors 53 and 54 caused by a leak is a considerable problem. This is a problem particularly when the capacitors 53 and 54 are OFF for a long time such as at the time of special reproduction.
At the time of special reproduction, a time constant for reproduction color burst level detection can be small since noise is ignored even if it is conspicuous. Therefore, a third capacitor is usually provided for storing level detection output of a color burst for special reproduction.
The discharge caused by a leak when a capacitor for storing a control signal is not used is a problem similarly in case of other electronic apparatuses and communication apparatuses as well as in the above-described case of VCRs.